Foam Wing Construction

The use of expanded bead
foam for construction of R/C aircraft revolutionized the model industry several years ago.
Use of this material began as a substitute method of building wings. This is its primary
use today but it is also used as a mold material for making fiberglass parts. It has
proven its value both in the shop and in the field because of certain advantages it has
over conventional materials. It is light weight, strong, easy to work with, and
inexpensive. One of the main advantages of the material is the speed and accuracy of
construction which is possible through its use.

Polystyrene foam is available in
densities ranging from .8 pounds per cubic foot to over twenty pounds per cubic foot. It
is normally found in white, blue, or gray. The type and density of the foam that is chosen
depends on the purpose that it will serve. Thick, light weight foam wings are normally
made of white foam while in thicknesses from 1/2" to 6 and is used in
construction of commercial buildings. Many building supply stores carry foam blocks in
appropriate sizes. Often, blue foam is used for rudders, stabilizers, or control surfaces
where the covering material may be thinner.

Any foam cutting systems requires two
basic components:

A frame or bow which is used to hold the cutting wire taut and guide the wire while
cutting

An electrical power supply which is used to supply the power required to heat the wire

The quality of a core is not
dependent on the complexity or the cost of a cutting system.

There are many different ways that a wing
can be made. Adding to this the different methods of foam construction, the building is
only limited by imagination. Below are diagrams which show two (2) typical wing cross
sections.

Due the vast amount of space that would
be required to attempt to cover this subject thoroughly, only the basics will be
covered.

Cutting Bow

Most of the foam cutting equipment in use
today is constructed by the home craftsman. This equipment is inexpensive and can be built
from readily available parts and materials. The design that is featured here is light
weight and easy to handle and is capable producing quality cores. This is by no means the
ultimate design and can be modified or completely redesigned to suit the individual
modeler.

The cutting frame or bow is nothing more
than a device for holding the cutting wire taut. A bow can be made by bending a piece of
conduit or gluing a series of dowels together. The bow can be as simple or as complicated
and the builder wants provided there is a method of holding the wire at the correct
tension. Basically, the bow should be light weight, easy to handle, and safe to use.

There are two basic methods of cutting
foam. The first method is to suspend the bow above work surface with a counterbalance as
shown in Figure 1. The counterbalance is nothing more than a cloth sack filled with sand
so that the bow can be raised to any position and it will stay at that position. This
leave the operators hands free to control the movement of the wire.

The second method is to allow the bow to
hang below a cantilevered working surface as shown in Figure 2. The advantage of this
method is that the bow is below the work surface and out of the way and the weight of the
bow can aid in setting the wire tension. The disadvantage is that a special work surface
must be constructed. The method used is up to the operator depending of space available
and personal preference. Either or both methods can be used depending on the core that is
to be cut. The same bow and work surface can be used in either method by attaching a
connector to the sash cord so the counterweight can be quickly disconnected. Each method
has advantages which can only be determined by use of both methods.

The design of the bow shown in Figure 3
is light weight yet very strong due to the I-beam construction and the plywood materials.
The tension of the wire is maintained by the 3/16 diameter music wire so that
adjustment is not required except when the wire is initially installed. This bow is
capable of cutting wings up to 60 span. If longer wing spans are required, the
length of the bow can be increased.

Construction of this frame is relatively
simple and is worth the effort for the strength and light weight. The main part of the
frame is constructed from 1/4 (6 mm) plywood. One piece is cut 1 1/2 x
30 (4 cm x 76 cm) and two pieces are cut 1 x 30 (2.5 cm x 76 cm). The
two narrower pieces are glued to the edges of the wider piece to form an I-beam. The
1 (2.5 cm) Dia. dowel pieces at each end are optional. The dowel can be split and
glued to each side of the center rib of the I-beam. The only purpose for these is to give
more strength where the holes are drilled for the music wire. Finally, 2 - 3/16 (5
mm) Dia. holes are drilled at a 20 degree angle, 1 1/16 (2.7 cm) in from each end of
the I-beam. The music wire provides the spring tension to keep the cutting wire taut. Two
(2) pieces of 3/16 (5 mm) Dia. music wire are cut to 9 (23 cm) lengths. The
end of the wires is slotted with a Dremel cutting wheel to a depth of about 1/8 (3
mm). The slot must be deburred and smoothed so that there are no sharp edges that will cut
the cutting wire. The wires should be forced in the drilled holes in the I-beam as shown
in Figure 3. Two (2) handles are made by drilling a 1/16 (1.5 mm) hole lengthwise
through the 1/2 Dia. x 1 1/2 (13 mm x 38 cm) dowel piece. These pieces are the
handles for controlling the hot cutting wire. The diameter and length of the piece is not
critical and can be made to suit the operator.

The cutting wire is made from .033
(.8 mm) Dia. nichrome wire. The length of the wire is determined by measuring the distance
between the music wire pieces at the widest point and adding about 1 (2.5 cm) to
make the wire easier to handle. One end of the wire is wrapped two (2) turns around a
brass eyelet and the eyelet is staked closed. The dowel handles are placed on the wire and
the same preparation is done to the opposite end. The finished wire assembly should be
about 1/2 (13 mm) less than the measured distance for proper tension. The wire is
inserted in the slots in the music wire pieces with the eyelets to the outside and the
handles to the inside. The connection to the cutting wire is dependent on the power source
that is being used. An extension cord that is of sufficient length to connect between the
bow and the power source can be used. The cord can be permanently attached to the brass
eyelets. The preferred method of attachment is to fix alligator clips to each lead of the
wire and connect these to the eyelets. The cord can be attached to the bow so that it will
not interfere with the cutting process.

Power Supply

The quality of a wing core is directly
dependent on maintaining the proper cutting temperature. This means that the power supply
is the most critical part of the system. The power supply may be as simple as an
automobile battery or as complicated as an adjustable step down transformer system. The
most common and probably the most effective power supplies in use today are inexpensive
automobile battery chargers. Another device in common use is the older larger type model
train transformers. The total investment for a power supply should not be more that
fifteen to twenty dollars.

There are several options available in
selecting a power source. A step down transformer that reduces the 115 volt household
power can be used. A model train transformer with adequate output power is an excellent
adjustable power source. Inexpensive automobile battery chargers are commonly used but the
most common source of power is the average automobile battery. It is best to use a power
source that has a voltage in a range of 6 to 15 volts for safety reasons. Some people have
connected household power through a light dimmer switch to a cutter and have been
successful but this is not recommended due to the inherent danger involved.

Additional Equipment

Some additional equipment may be required
depending on the type of wing cores that are being cut. If the cores are to be beveled on
the end for dihedral, a box to hold the core and templates is required or the bevel
templates can be fixed to the sides of the core. This will be discussed in more detail
later.

A simple cut-out tool will be required
for cutting recesses for landing gear blocks, aileron servos, pushrod channels, etc. Tools
can be made by bending 14 gage solid wire to the appropriate shape and inserting this in a
soldering gun. The use of these tools will be explained and this will clarify how the
tools are made.

Templates

The most important item in the foam
cutting process is the template. The resulting core can be no better than than the
templates that are used to produce the core. The shape and finish of the core is directly
related to the shape and finish of the template. The shape of the template is dependent on
the wing construction, i.e.. whether trailing edge stock is used, whether strip or barn
door ailerons are used, whether spars are used, etc.

The material used for the templates is a
matter of preference. Almost any type of material which is heat resistant, durable and
easily worked is satisfactory. Probably the most common material used is plywood because
it is readily available. One of the best materials is Formica or any of the common cabinet
top materials. This is available from cabinet shops and quite often scraps of sufficient
size will be given away. Thin aluminum works well but may act as a heat sink at the ends
of the cores causing the wire to drag.

The rib outline can be traced or copied
and then transferred to the template material. Again, the method used is a matter of
preference. Carbon paper can be used to trace the outline. A xerographic copy can be
ironed onto the material. The copy or tracing can be glued to the material. The only thing
that matters is that an accurate outline of the section be transferred to the material so
that it can be cut, shaped, and finished to match the rib section of the wing.

Two templates must be made, one for the
root and one for the tip. A constant chord wing requires two identical templates while a
tapered wing requires two of the appropriate size and and section. The templates are
numbered on both sides as shown in Figure 4.

The marks are used to guide the operators
in maintaining a constant cutting speed throughout the cord. The numbers are closer
together where the curvature is more critical and near the end of the template. For
semi-symmetrical and symmetrical wing sections, both edges of the template must be
numbered.

The method of making the template is
dependent on the material used and personal preferences. The drawing for the template can
be glued to the template material with a spray adhesive such as 3M-77. The template is
then cut just outside the outline. Final shaping is done by sanding the template to the
outline. The finish must be smooth with no ripples or rough spots that could cause the
cutting wire to catch or drag. Holes are drilled in the template for nails that are used
to attach the template to the foam. The holes must be snug so that the template does not
move during coring.

Preparation

In order for the foam wing cores to be
accurate, the foam block must be square. This is relatively easy to accomplish but the
importance of this task can not be overstated. Care in squaring the block at this point
will result in reduced labor or inaccuracy later in the final stages of construction. The
recommended tools for this procedure are:

A cutting bow

An electrical power

Metal square

Metal rule (longer than foam block)

Two (2) square templates

Nails to hold templates to foam block

The work are should be clean and orderly.
The cutting bow and power supply are connected and allowed to begin heating. The foam
block is placed on the cutting table and may be attached with double stick tape, weighted,
or simply held in place during trimming and squaring.

Squaring of the foam block starts with
the ends being squared. This is the most important step. A metal square is used to
determine if either end of the foam block is square. If it is then this is a good starting
point. Otherwise, both ends of the block must be squared.

The metal square is used to draw lines
across the top face of the block then the ends of this line is drawn down the sides of the
block. These lines are the guides for locating the square templates. The numbered edge of
the square template is aligned with the line and nails are pressed through the holes into
the foam block. The end of the block is then cut off using the normal cutting process
described later.

After the foam block has been squared, it
is then cut to size. This is done using the same method used in squaring the foam. The
core width is measured, the guide lines are drawn, and the square templates are located on
the guide lines. For tapered wings, the core can be cut to the finished taper with a width
equal to the finished core with at each end.

Cutting

The templates are attached to the foam
using nails with large heads such as roofing nails. Locating the templates accurately is
essential to the quality of the finished cores. First, the root template should be located
so that adequate stock is left on the top and bottom so that the cutting wire does not
burn through the outer surface. If the wing has dihedral, the tip template can be located
sufficiently so that the core is cut with built-in dihedral.

The Numbered Template diagram shows the
leading edge and trailing edge as black filled areas. These will overhang the core stock.
Measurements are made from the bottom edge of the foam to the center of the template so
that the centerline is exactly parallel to the bottom edge. The nails are pushed through
the holes in the template into the foam. After the root template is mounted, it should be
rechecked for accuracy.

The tip template is located in a manner
similar to the root template except that the centerline may be above the centerline of the
root template to account for the dihedral. Wash-in or wash-out can be cut into the core by
raising or lowering the trailing edge of the tip template by the appropriate amount.
Again, the template should be checked for accuracy before the cutting is started.

When the templates are attached, the foam
is made ready for cutting. It is not necessary to mount the foam to the cutting table but
this is an option of the builder. It may be attached with double sided tape to avoid
having the block move during coring. The bow is placed on the cutting table and allowed to
heat to the cutting temperature. The wire is tested with scrap foam to assure that the
proper temperature is reached.

Although many people have developed the
skills to cut cores alone, cutting the core is essentially a two person operation. One
person sets the pace of the wire movement but calling out the numbers on the template as
they are passed while the other person follows the lead. The follower must adjust his
movement according to the instructions of the leader gradually. Sudden jerking or stopping
to compensate with result in wire lag or over burning. Smooth movement is essential to
cutting a smooth, useable core.

The cutting operation begins with the
operators placing the hot wire on the template overhang adjacent to the foam. When the
leader is ready, he says, "Now" and both operators move the wire to the foam at
the same time. The leader begins to call out the template numbers as each one is reached.
The wire must be allowed to do the cutting. Forcing the wire through the foam will
result in wire lag or tearing of the foam. Either condition will render the core unusable.
Both operators must reach the leading edge of the foam at the same time. When the leader
sees that his end of the wire breaks free of the foam, he says, "Out" so that
the follower knows that the operation is complete. The bow is placed in a safe place and
the power is turned off. The core will be left in the cradle while the opposite of the
core is cut which is done in the same manner.

Tapered wings present a special problem
that requires special attention from the operators. This type core requires that the wire
end at the tip move faster than the end at the root to avoid burning or over-melting. It
is easier to feel and correct wire lag at the faster moving end so the leader should be
cutting at the tip. The operators should be constantly aware of the feel of the cutting
operation and communicate constantly.

Not all wings fall in the category of
straight or fixed taper wings. Wings such as those for a Corsair present the final problem
to the operator; how to make a multi-taper polyhedraly wing. Since the wire can only cut
in a straight line, this type of wing must be made in sections and joined after covering.
The detail shows how a Corsair wing might be made. Section 1 is made with templates A and
B while Section 2 is made with templates B and C. This method can be carried over into
making any number of sections with varying tapers and dihedrals to form a wing of any
shape which is made up of a series of straight lines.

Because the foam is very fragile and
susceptible to physical and chemical damage, it should be left in the foam cradles from
which it is cut until time for covering. The cradles are used to protect the finished core
from physical damage and from dust accumulation that could adversely affect the adhesion
of the sheeting. They are also used during sheeting to apply force to the material until
the adhesive is set. If the templates for the adjoining sections are properly positioned,
the cradles can be used to accurately join the wings and hold them while the adhesive
sets. For these reasons, care must be taken in the handling to the cradles as well as the
cores.

Sheeting

There are many different materials which
can be used to sheet a foam core from brown wrapping paper to thin plywood depending on
the type of model with which the wings will be used. For each type of sheeting, there may
be several different methods of joining the sheeting to the core. One method or material
may be ideal for one application but be totally inadequate for another. Since there are so
many different methods and materials, only the most common method will be addressed.

Each of the most common sheeting
materials have a special application and require different materials and skills to use.
The lower strength, lighter weight materials are more commonly used for sailplane wings
while the stronger, heavier materials are used for pattern or racing planes. Balsa is
still the most common material for building models and this holds true for sheeting foam
wings.

COMMON SHEETING MATERIALS

MATERIAL

ADVANTAGES

DISADVANTAGES

Balsa Sheet

Light weight, strong, readily available, easy to form,
easy to sand

Relatively expensive, fragile, not available in adequate
widths

Obechi Plywood

Very strong, hard, easy to form, easy to finish,
available in large sheets

Inexpensive, light weight, readily available in large
sheets, easy to form

May absorb moisture or chemicals, difficult to repair,
low strength

Fiberglass Cloth

Very strong, readily available in large sheets, easy to
form, easy to finish

Expensive, heavy, requires special equipment and skills

Since balsa sheeting is by
far the most common sheeting material, the method for applying balsa sheeting will be
covered. Some of the practices will also apply for other covering material such as Obechi
plywood and poster board while wrapping paper and fiberglass require significantly
different practices.

The balsa sheets must first be prepared
for use as a sheeting material. Since it is not available in large sheets, the narrow
sheets must be butt joined to form a single wide sheet. Assuming that the wing panel has a
semi-span of 32" and a chord of 12", the sheeting width required will be
10" allowing for a 1/2" leading edge cap and a 1 1/2" trailing edge strip.
If the cores are tapered, a double width sheet can be prepared to reduce the amount for
waste. For example, if a wing has a 12 in root chord and 8" tip chord, two sheets
10" wide or one sheet 16" wide can be prepared. Cutting the 16" wide sheet
to the proper taper will reduce the amount of waste by 4". The edge of the sheets
must be trimmed straight using a metal straight edge and an Xacto knife or razor
blade. Every edge that will be butt joined to form the sheet covering must be trimmed.

After pieces are trimmed, they must be
butt joined to form a sheet of the appropriate width. The pieces are laid edge to edge on
a flat surface and a strip of masking tape is placed over each joint to hold the edges
tightly together. Then the sheet is turned over and raised so that the joint is opened so
that glue can be applied to the edges. A good aliphatic resin glue, such as Titebond, is
applied to the mating edges. When all edges have had glue applied, the sheet is laid with
the taped surfaces down on a flat surface. The excess glue is wiped with a wet paper towel
or rag. This will reduce the amount of effort required to clean the surface later. The
sheet is left to dry according to the adhesive manufacturer's instructions. Weight can be
applied to the sheet to hold it flat if desired.

After the adhesive has cured, the sheet
should be sanded to remove any ridges caused by swelling of the material at the glue
joint. It is much easier to remove the ridges at this stage than after the sheeting has
been applied to the core. The outer surface can be finish sanded to reduce the effort
required when preparing for the finish.

There are three adhesives which have
gained wide acceptance for attaching sheeting to cores; 30 minute epoxy, 3M77 spray
contact adhesive, and Southern Sorghum foam core adhesive. The 3M77 and Southern Sorghum
are more difficult to work with since they require that the sheeting be place exactly in
the correct position the first time. The 30 minute epoxy is much easier to work with by if
not done properly will cause a significant increase in the weight of the wing. The best
way to apply the adhesive is to follow the manufacture's instructions. If epoxy is used,
it must be applied sparingly. A plastic squeege must be used to remove as much of the
epoxy as possible before the sheeting is applied to the core. The contact adhesives must
be allowed to cure as shown in the instructions to avoid trapping vapors under the skin
and causing damage to the core.

Regardless of the adhesive used,
application of the sheeting is the same. One edge of the sheeting is placed flush with the
trailing edge of the core while the sheet is held away from the core. When the trailing
edge has been aligned properly, the palm of the hand is used to press the sheeting down on
the core working slowly toward the leading edge. When the leading edge is reached, there
should be a small amount of excess shheting overhanging which will be trimmed later.

After the sheeting is applied, the core
assembly is returned to the bottom cradle and the top cradle is placed over the assembly.
When the core assembly and the cradles are properly aligned, a large amount of weight is
distributed evenly over the top cradle. The core assembly should be allowed to stay in the
cradles for a minimum of 24 hours to allow total curing of the adhesive. Epoxy and contact
adhesives continue to cure even after the cemented surfaces are joined.